The so-called “lost-foam” casting process is a well-known technique for producing metal castings wherein a fugitive, pyrolizable, polymeric foam pattern is covered with a thin, permeable, refractory coating, and embedded in a mold formed of unbonded refractory particles (e.g. sand) to form a molding cavity within the bed of particles. Metal melt, e.g., aluminum or magnesium, is then introduced into the mold cavity to pyrolize the foam, and displace it with melt. Gaseous and liquid pyrolysis products escape the molding cavity through the permeable refractory coating into the interstices between the unbonded refractory particles. The most popular polymeric foam pattern comprises expanded polystyrene foam (EPS) having densities varying from 1.2 to 1.6 pounds per cubic foot. Other pyrolizable polymeric foams such as polymethylmethacrylate (PMMA), and copolymers are also known. The melt may either be gravity-cast (i.e. melt is poured from an overhead ladle or furnace), or countergravity-cast (melt is forced upwardly e.g. by vacuum or low pressure) into the bottom of the mold from an underlying vessel.
In gravity-cast lost-foam processes, the metallostatic head of the melt is the driving force for filling the mold with melt. Gravity-cast lost-foam processes are known that (1) top-fill the mold cavity by pouring the melt into a basin overlying the pattern so that the melt enters the mold cavity through one or more gates located above the pattern, or (2) bottom-fill the mold cavity by pouring the melt into a vertical sprue that lies adjacent the pattern and extends from above the mold cavity to a gate(s) at the bottom of the mold cavity for filling the mold cavity from beneath the pattern. According to one countergravity-casting technique, known as “low pressure lost-foam casting”, melt is contained in a crucible that is contained within a sealed vessel that underlies the mold. A filler-tube extends upwardly from within the melt in the crucible to the gate of an overlying, bottom-gated, unbonded refractory particle mold. When the vessel is pressurized (e.g. with nitrogen), melt rises up the filler-tube and into the mold cavity, displacing the pyrolizable foam therein and filling the molding cavity. In low-pressure lost-foam casting, the driving force for moving the melt into the mold is gas pressure applied to the sealed vessel containing the crucible.
It is known to provide one or more unobstructed, foam-free, melt flow-channels, or shafts, in the pattern through which the melt can rapidly flow directly to selected regions of the pattern. Such melt flow-channels are often called “lighteners”, and are commonly formed at the joints between individual pattern segments that are joined together to form a single pattern, or as interconnected internal voids that transect the segments. Lighteners may also be formed by molding the foam pattern around an insert (e.g. a rod) and subsequently withdrawing the insert from the pattern to leave a foam-free shaft.
It is known to use chills with empty-cavity casting processes to locally cool a region of a casting in the vicinity of the chill at higher rates than other regions of the casting are cooled in order to reduce porosity, refine the microstructure and enhance the physical properties of the casting. The use of chills with lost-foam casting has also been proposed. For example, Ryntz Jr. et al. U.S. Pat. No. 4,520,858, which is assigned to the assignee of the present invention, and hereby incorporated herein by reference, glues the cooling face of a chill directly onto the surface of an EPS foam pattern using an adhesive that vaporizes under the heat of the melt. Chills are made from materials, such as metals, that have high thermal diffusivities (i.e. the quotient of the division of the material's thermal conductivity by the product of its specific heat times its density), which is a measure of the ability of the material to absorb heat. Copper, cast iron and graphite are known to be suitable chill materials for casting aluminum, and may be water-cooled for added effectiveness. The amount of heat a chill can absorb is also a function of the mass of the chill (i.e. larger chills can absorb more heat).
Lost-foam castings made from molds having chills whose cooling faces contact the pattern develop a rough surface on the casting at the site where the chill engages the casting. In this regard during casting, liquid pyrolysis products from the pyrolysis of the foam pattern become trapped between the advancing metal front and the cooling face of the chill where they are transformed into large volumes of gas that cannot escape through the chill. Rather, they are forced to vent along the interface between the chill and the melt, or into the melt adjacent the interface, which creates a rough surface characterized by a heterogeneous assortment of shallow hills and valleys similar in appearance to a water-eroded surface [e.g. see FIG. 4(c)]. The rough surface not only detracts from the appearance and utility of the casting, but also can reduce the heat transfer between the melt and the cooling face.